Apparatus for making membranes



Dec. 12, 1967 I K. D. vos

' APPARATUS FOR MAKING MEMBRANES Filed Sept. 2, 1965 [.22 v52: for Q-WA/Em Q 1 05 W'1 iwltkailm, H14, v 5

United States Patent 6) 3,357,052 APPARATUS FOR MAKING MEMBRANES Kenneth D.--Vos,' San Diego, Calif, assignor to General Dynamics Corporation, New York, N.Y., a corporation of Delaware Filed Sept. 2, 1965, Ser. No. 484,515 9 Claims. (Cl. 18-15) ABSTRACT OFTHE DISCLOSURE- wherein controlled evaporation of solvent occurs from the exposed surface of the film deposited on a cylindrical drum before the film enters a water bath wherein the lower portion of the casting drum rides.

This invention relates to semipermeable membranes and more particularly to apparatus for continuously casting a semipermeable membrane from a fluid casting solution.

semipermeable membranes, which permit the passage therethrough of a solvent, while rejecting a solute, when a solution at least a certain minimum pressure is applied to the membrane, can be cast from fluid casting solutions, such as cellulosic ester solutions. Such semipermeable membranes are sometimes hereinafter referred to as nonuniform membranes because one surface has extremely different characteristics than the other surface. This one surface is sometimes termed the active surface layer, often only about 0.2 micron in thickness, and is believed to account for the osmotic properties of the membrane. The process by which some such semipermeable membranes operate is often referred to as reverse osmosis. One example of a nonuniform semipermeable membrane of this type is more particularly described in United States Patent No. 3,133,132 to Loeb et al.

In the general process described in this patent, a casting solution is prepared by dissolving a film-forming cellulosic ester, such as cellulose acetate, plus an aqueous solution of a pore-producing salt, in an organic solvent.

This'casting solution is cooled to a temperature about -7.5 to -16 C., and a plate is cooled to about the same temperature. A thin film of the solution is then cast between runners on the plate, and the thickness of the cast film is held to a uniform dimension by dragging a doctor blade across the runners. Evaporation of the organic solvent is permitted for a predetermined time period. After this time, the plate and cast film are immersed in water at a temperature slightly above the freezing point.

The above-described casting process is inherently a batch-type operation and sulfers' the disadvantages of any batch-type operation. More economical processes for manufacturing membranes of this type are desired.

An improved, continuous process for casting semipermeable membranes is disclosed in copending US. patent application Ser. No. 469,419, filed July 6, 1965, in the name of Ulrich Merten and assigned to the assignee of this-case. In this process, a continuous length of semipermeable membrane is'produce'd'by continuously casting athin film of a'flu'id casting solution on a moving endless 7 3,357,652 Patented Dec. 12, 1967 flat surface. The'cast film is exposed for a'predetermined short period of time to anatmosphere which allows limited evaporation of'a solvent component of the casting solution. to occur, the exposure aiding in the initial organization of the active'surface layer of the nonuniform membrane. The cast film is then brought in contactwith a liquid to complete the organization of the semipermeable membrane;

Apparatus for economically performing this process is also disclosed which includes amoving endless fiat surface, such as a rotating cylinder, which is driven at a timed rate of'linear surface travel, and which is disposed so that a thin film of a fluid casting'solution may be cast upon the upper surface. The rate of travel of the endless surface exposes the'cast film to the atmosphere adjacent thereto for a predetermined period, and then enters a region wherein the cast film comes in contact with a liquid which completes the organization of the membrane. The apparatus also includes means for removing the com pleted membrane from the moving endless surface before it reaches the location where casting takes place.

It is an object of the present invention to provide an im proved apparatus for castingsemipenneable membranes. It is another object to provide apparatus for casting a continuous semipermeable membrane from a fluid casting solution. It is a further object to provide apparatus for producing thin semipermeable membranes from cellulosic ester solutions in such a manner that the chemical and physical properties of the cast membranes are precisely reproducible. It is a still further object to provide improved apparatus for economically casting thin nonuniform semipermeable membranes. These and other objects of the invention are more particularly set forth in the following detailed description and in the accompany ing drawings wherein:

FIGURE 1 is a diagrammatic view of apparatus for casting continuous lengths of semipermeable membranes which apparatus embodies various features of the present invention.

FIGURE 2 is an enlarged fragmentary sectional view taken vertically through the apparatus shown in FIGURE 1; and

FIGURE-3 is an enlarged fragmentary view of a portion of the apparatus shown in FIG. 2.

An improved device for feeding a fluid casting solution is provided for use inapparatus for the continuous casting of thin semipermeable membranes. The improved feed device allows precise and easy application of a thin film of casting solution and thus provides for excellent reproducibility of results in the physical properties of the membranes produced on this apparatus.

The invention, for ease of description, is hereinafter disclosed with reference to the casting. of a semipermeable membrane from a cellulosic ester solution. HOWVr, it should be understood that the apparatus disclosed herein may be employed in making other similar types of semipermeable membranes.

A continuous casting apparatus 11 is illustrated in FIG- URES 1 and 2 of the drawings. The apparatus 11 generally comprises a cylindrical drum 13 which provides a smooth flat endless casting surface, which drum is rotatably mounted within an open-top tank 15. A generally V-shaped casting trough 17 is adjustably disposed above the top of the rotatable drum 13 and serves to apply a thin film of a castingsolution to the rotating underlying drum 13, A take-up drum 19 is laterally spaced from the rotatable casting drum 13 and serves as the take-up reel for the continuous length of cast membrance.

Drive means 21 and 23 are provided for rotating the drums 13 and 19, respectively, in timed relation to each other. A water circulation system (not shown) assures that the tank 15 is kept filled with water, preferably distilled water although tap water may be used, to the desired level, usually up to a vertical level at about the middle of the drum 13, and that there is a sufficient change of the water in the tank 15 so that no undesirably high concentrations of substances build up therein. To suitably regulate the temperature of the water within the tank 15, a cooling system (not shown) is provided.

The casting drurn 13 is mounted on a horizontal shaft 25 which extends through the sidewalls of the tank 15 and is supported in a pair of bearings attached to the outer surfaces of the sidewalls. Shaft seals are provided to prevent water from entering the bearing housings. At one end, the shaft 25 extends through the bearing housing; to this extension of the shaft 25, the drive means 21 is attached.

The ,V-shaped casting trough 17, which is mounted vertically above the casting drum 13, is supported by a pair of upright standards 27 which are bolted to the outer surfaces of the sidewalls of the tank 15. Each of the standards 27 includes a vertical slot 29, open at its top, wherein the trough 17 is restrained against lateral movement while vertical movement of the trough is permitted. The casting trough 17 comprises a rear wall 31 and a pair of parallel triangular shaped sidewalls 33 which are fixedly attached to the rear wall 31. The term rear as used herein, refers to the position of the wall of the trough 17 relative to the direction of rotation of the casting drum 13, counterclockwise as shown in FIGURES 1 and 2. The casting trough 17 is completed by an adjustable forward wall 35 which is spaced from the rear wall 31 at the lower portion thereof to provide an opening through which the casting solution flows downward onto the drum 13. A transparent plastic cover 36 closes the top of the trough 17.

The trough 17 is supported above the casting drum 13 by a pair of laterally extending support arms 37, mounted to the outer surfaces of the triangular sidewalls 33, which arms are proportioned to laterally fill the vertical slots 29 and thus stably support the trough 17 therein. The weight of the trough 17 plus a pair of coil springs 39 retain the support arms 37 within the slots 29. The ends of each spring 39 are respectively attached to a point at the end of one of the support arms 37 and to a fixed point below on the outer surface of one of the standards 27.

The bottom of the fixed rear wall 31 of the trough rides on the surface of the casting drum 13. To prevent any leakage of casting solution rearward from the trough 17 down the casting drum surface and to reduce the drag of the trough on the drum 13, the rear wall 31 includes a lower portion in the form of a wiper blade 41. The blade 41 is made of a resilient material, such as Teflon, which has low friction characteristics. The bottom edges of the sidewalls 33 are contoured to match the cylindrical surface of the drum 13. Because the blade 41 rides on the upper surface of the casting drum 13, it accordingly supports the trough 17 atop the casting drum. The thickness of the film being cast is regulated by the adjustable forward wall 35. Outward leakage from the trough under the sidewalls 33 is prevented by the small clearance pro vided between the bottom of the sidewalls 33 and the drum 13 in combination with the inherent high viscosity of the casting solution.

The adjustable forward wall 35 is supported in any suitable manner so that its bottom edge can be easily moved toward and away from the surface of the casting drum 13 to thereby adjust the distance therebetween to regulate the fiow of the casting solution from the trough 17 onto the rotating drum. In the illustrated embodiment, a pair of slotted guideways are milled out of the inner surfaces of the triangular sidewalls 33. The lateral edges of the forward wall 35 reside in these guideways. The edges of the forward wall 35 may be provided with suitable seals, if desired, to prevent undesired leakage of the casting solution out of the trough 17 through these guideways.

To secure the slideable forward wall 35 in the desired relative position, a pair of clamping screws 43 are provided which pass through slots 45 provided in the front wall at a location above the normal level of casting solution in the trough 17. The clamping screws 43 reside in tapped holes in brackets 47 affixed to the inner surface. of the triangular sidewalls 33. By tightening the clamping screws 43, the forward wall 35 is clamped at any desired position in the guideways. As is clear from FIG. 2, the height of the bottom surface 51 of the forward wall 35 above the surface of the casting drum determines the thickness of the strip of membrane which is produced. The viscosity of the casting solution is sufficiently high relative to the thickness of the thin film that there is no significant sideway flow on the casting drum surface from the unrestrained sides of the film of casting solution as the film emerges from the casting trough 17.

Because of the thickness of the semipermeable membrane usually produced, e.g. cellulose acetate membranes about microns thick are adequate, careful regulation of the thickness of the film applied is very important. Moreover, to assure reproducible results from different production runs, the thickness of the film of casting solution should be closely regulated. It has been found that the bottom surface 51 of the front wall 35 should be smooth and should be located in converging relationship to the coating drum 13. By converging, it is meant (see FIG. 3) that the distance (x) between the rear edge 52 of the bottom surface and the drum 13 is greater than the distance (y) between the forward edge 53 and the drum. The smoothness of the bottom surface 51 assures that a very smooth upper film surface is provided. Preferably, the smoothness of the bottom surface 51 is at least about 10.0003 inch.

If the flat bottom surface 51 is disposed horizontally, as it preferably is, it is positioned on the uphill side of the crest of the drum 13 (as shown in FIG. 2), so that a slight compressive action occurs in the upper region of the film as a point on the curved surface of the rotating drum 13 moves under the horizontal bottom surface 51. The compressive action aids in establishing the reproducibility in the operating characteristics of the semipermeable membranes made by this apparatus 11. To obtain the desired amount of compressive action, the x to y ratio should be at least about 2 to 1.

In making a nonuniform semipermeable osmotic membrane, the treatment to which the extreme upper surface region is subjected becomes very important. The solute rejection properties which such a membrane possesses are believed to result from the establishment of an extremely thin layer, i.e. about 0.2 micron thick in a cellulose acetate membrane, which has a different density and construction than the remainder of the membrane. It is believed that this extremely thin layer is responsible for the rejection of the solute while it allows solvent molecules to freely pass therethrough.

It has been found that by the use of a trough 17 with the above-mentioned properties and with a wide opening (marked 0 in FIG. 2) between the adjacent edges of the front wall 35 and the rear wall 31, relative to the thickness of the film being deposited, membranes are produced which consistently have acceptable operating characteristics. Although the precise technical effect of the width of the opening is not entirely understood, it is believed that the employment of an opening of such a width permits a generally static film region to be established adjacent the surface of the drum before the rotating drum passes under the thickness-limiting bottom surface 51. If an opening less than about 20 times the thickness of the film being cast is employed, the film region may still be in dynamic condition as it passes under the front wall 35, thus increasing the likelihood of potential variation in the resultant membrane. Accordingly, the opening employed is at least about 20 times the height of forward edge 53 of the forward wall 35 above the surface of the casting drum 13.

In one example of a suitable apparatus 11, a casting drum 13 having a smooth cylindrical surface made of stainless steel of a diameter of 12 inches is employed to produce a membrane about 100 microns thick. The casting drum 13 is driven at about revolution a minute, which is equal to a linear surface speed of about 45 feet per hour. The water level in the tank 15 is held at the level of the horizontal axis of the casting drum 13 so that the bottom half of the surface of the drum is immersed in the water. A take-up drum 19 having a cylindrical surface, made of polyvinyl chloride, about 2 /2 inches in outer diameter is spaced as illustrated in FIGURE 1, completely immersed in the tank 15. The take-up drum 19 is driven so that its surface has a slightly higher linear speed than that of the casting drum 13 to keep the membrane in tension during take-up, with sufiicient slip in the drive means 23 being provided to prevent the membrane from tearing.

The trough 17 is made of 0.5 inch thick stainless steel. The bottom surface 51 of the front wall is cut at an angle so that it is 0.562 in. wide. The bottom surface 51 is flat and is ground to a smoothness of about :00003 inch. The wiper blade 41 is made up of a strip of Teflon about 0.25 in. thick. The angle at which the bottom surface 51 is cut is such that when the front wall 35 is disposed in the guideways in the triangular sidewalls 33 and the trough 17 is mounted above the drum, the bottom surface 51 is horizontal. The front wall is set, using the clamp-.

ing screws 43, so that its forwardmost edge 53 is about 13 mils above the surface of the casting drum 13, the thickess of this film decreasing as the membrane sets up with the resultant product being about 100 microns thick. In this position, the edge 53 lies vertically above the crest of the drum 13 or just slightly to the right thereof as seen in FIG. 2. The horizontal opening (marked 0 in FIG. 2) between the lower portions of the front and rear walls 35, 31 is about 0.5 inch. The distance (x) from the rear edge 52 vertically to the surface of the drum is about 53 mils so that the x to y ratio is about 4.1.

The trough 17 is filled to the desired level with the precooled casting solution to provide a reservoir of solution, and the operation is ready to begin, and downward flow of the casting solution begins. A thin coating of the casting solution is supplied uniformly across the major portion of the surface of the stainless steel casting drum 13. The temperature of the drum surface approximates that of the water in the tank 15, -i.e. about 1 C. I

The cast solution is exposed to the air for about one minute from the time it is cast until it becomes immersed in the distilled water. Immersion in water at about 1 C. completes the set-up of the nonuniform membrane and removes the remaining organic solvent and the swelling agent. The set-up membrane is easily removed from the casting drum 13. After washing in a bath of circulating water, the membrane is examined and found to have a useful width of about 12 inches and a uniform thickness of about 100 microns.

Testing of samples of the membrane from different 10- cations along a length of the cast membrane shows excellent uniformity in salt rejection characteristics and also shows that the membrane produced is considered to be excellently adaptable for use in a separation application where rejection of a high percentage of solute from an aqueous feed solution is desired.

Although the invention has been described with reference to certain specific examples and materials, it should be understood that these do not constitute limitations upon the scope of the invention and that modifications which would be obvious to one skilled in the art are considered as coming within the scope defined by the appended claims.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. In apparatus for continuous casting of a semipermeable membrane from a fluid casting solution, which apparatus includes means defining an endless fiat smooth surface, means mounting said endless surface 'de-fining means for travel from a first. location where a filr'n of the casting solution may be applied thereto, past a second location where a volatile component maybe evaporated from the applied film, and subsequently to a third location where the applied film is contacted by a liquid, and means for driving said endless surface defining means at a predetermined speed, the improvement which comprises means for establishing a supply reservoir of the fluid casting solution above said endless surface-defining means at said first location, the hydraulic head of said reservoir bearing directly vertically upon said endless surface de fining means, said reservoir-establishing means having a fixed rear wall and an adjustable forward wall. which are spaced apart to provide an opening .therebetween', the lower portion of said rear wall engagingsaid endless surface-defining means and preventing rearward flow of casting solution therealong, and'means for spacing said adjustable forward wall a predetermined distance above said surface-defining means so that a thin film of casting solution flows onto said endless surface-defining means ,in the direction toward which said endless surface-defining means is driven, the bottom surface of said forward wall being disposed in converging relationship to said endless surface-defining means so that the rear edge of the bottom surface of said forward wall is spaced at least twice as far above said surface-defining means as the distance between the forward edge of said bottom surface and said surface-defining means whereby a compressive action is provided.

2. In apparatus for continuous casting of a nonuniform semipermeable membrane from a fluid casting solution, which apparatus includes a cylindrical casting drum having a smooth surface, means rotatably mounting said drum on a horizontal axis, means for providing a liquid bath in which the lower portion of said drum is immersed, and means for rotating said drum at a predetermined speed, the improvement which comprises means for establishing a supply reservoir of the fluid casting solution above said drum surface, said reservoir-establishing means having a fixed rear wall and an adjustable forward wall, spaced apart at their lower portions to provide an opening therebetween, the hydraulic head of said reservoir bearing directly vertically upon said drum surface through said opening, the lower portion of said rear wall engaging said drum surface and preventing rearward flow of casting solution therealong, and means for spacing said adjustable forward wall a predetermined distance above said drum surface so that a thin film of casting solution flows onto said drum surface in the direction toward which said drum rotates, the bottom surface of said forward wall being smooth and horizontal and being located on the uphill side of the crest of said drum so that compressive action occurs at the upper surface region of the applied film be tween the cylindrical drum surface and the horizontal bottom surface of'the forward wall, the height of the rear edge of said bottom surface above said drum surface being at least about twice the height of the forward edge of said bottom surface above said casting drum surface.

3. In apparatus for continuous casting of a nonuniform semipermeable membrane from' a fluid casting solution, which apparatus includes a cylindrical casting drum having a smooth surface, means rotatably mounting said drum on a horizontal axis, means for providinga liquid bath in which the lower portion of said drum is immersed, and means for rotating said drum at a predetermined speed, the improvement which comprises means for establishing a supply reservoir of the fluid casting solution above said drum surface, said reservoir-establishing means having a fixed rear wall and an adjustable forward wall which walls are spaced apart at their lower portions to provide an opening therebetween, the hydraulic head of said reservoir bearing directly vertically upon said drum surface through said opening, and the lower portion of said rear wall engaging said drum surface and preventing rearward flow of casting solution therealong, and means for spacing said adjustable forward wall a predetermined distance above said drum surface so that a thin film of casting solution flows onto said drum surface in the direction toward which said drum rotates, the bottom surface of said forward wall being smooth and horizontal and being located on the uphill side of the crest of said drum so that compressive action occurs at the upper surface region of the applied film between the cylindrical drum surface and the horizontal bottom surface of the forward wall, the dimension of said opening between the most proximate points on the lower portions of said rear wall and said forward wall being at least about 20 times the height which said bottom surface of said forward wall is spaced above said casting drum surface.

4. In apparatus for continuous casting of a nonuniform semipermeable membrane from a fiuid cellulosic ester casting solution, which apparatus includes a cylindrical casting drum having a smooth surface, means rotatably mounting said drum on a horizontal axis, a water bath in which the lower portion of said drum is immersed, and means for rotating said drum at a predetermined speed, the improvement which comprises a V-shaped trough holding a supply reservoir of the fluid casting solution above said drum surface, said trough having a fixed rear wall and an adjustable forward wall which are spaced to provide an opening through which the hydraulic head of said reservoir bears directly vertically upon said drum surface, the lower portion of said rear wall engaging said drum surface and preventing rearward flow of casting solution therealong, and means for spacing said adjustable forward wall a predetermined distance above said drum surface so that a thin film of casting solution flows onto said drum surface in the direction toward which said drum rotates, the bottom surface of said forward wall being smooth and horizontal and being located on the uphill side of the crest of said drum so that compressive action occurs at the upper surface region of the applied film between the cylindrical drum surface and the horizontal bottom surface of the forward wall, the dimension in FIGURE 2 of said opening being at least about times the height of said bottom surface above said drum surface, and the height of the rear edge of said bottom surface above said drum surface being at least about two times the height of the forward edge thereof.

5. In apparatus for continuous casting of a thin porous membrane sheet from a fluid casting solution of organic material, which apparatus includes a cylindrical casting drum having a smooth surface, means rotatably mounting said drum on a horizontal axis, means for establishing a supply reservoir of the fluid casting solution above said drum surface, said reservoir-establishing means having a rear wall and a forward wall spaced apart at their lower portions to provide an opening therebetween, means for spacing said forward wall a predetermined distance above said drum surface so that a thin film of casting solution flows onto said drum surface in the direction toward which said drum rotates, and means for rotating said drum at a predetermined speed, the improvement which comprises said reservoir-establishing means being disposed so the hydraulic head of said reservoir bears directly upon said drum surface through said opening, the bottom of said forward wall being smooth and substantially flat and being located on the uphill side of the crest of said drum so that compressive action occurs at the upper surface region of the applied film between the cylindrical drum surface and the bottom surface of the forward Wall, and the height of the rear edge of said bottom surface above said drum surface being at least about twice the height of the forward edge of said bottom surface above said casting drum surface.

6. The invention in accordance with claim 5 wherein said reservoir-defining means is disposed with said bottom surface of said forward Wall horizontal.

7. The invention in accordance with claim 5 wherein said reservoir-defining means is disposed with said bottom surface of said forward Wall above said drum surface a sufficient distance to cast a membrane no greater than about microns in thickness.

8. In apparatus for continuous casting of a thin porous membrane from a fluid casting solution of organic material, which apparatus includes means defining an endless flat smooth casting surface, means mounting said endless surface-defining means for travel along a path wherein said casting surface is made up of lines which are parallel to the horizontal plane, means for driving said endless surface-defining means at a predetermined speed, means for establishing a supply reservoir of the fluid casting solution above said path, said reservoir-establishing means having a rear wall and a forward wall spaced apart at their lower portions to provide an opening therebetween, the improvement which comprises means for mounting said reservoir-establishing means with the hydraulic head of said reservoir bearing directly upon said endless surface-defining means through said opening and with said forward wall spaced a predetermined distance above said endless surface-defining means so that a thin film of casting solution flows onto said endlesssurface in the direction toward which said endless surface-defining means is driven, the bottom surface of said forward wall being smooth and substantially fiat and being disposed in converging relationship with said endless surface-defining means so that compressive action occurs at the upper surface region of the applied film between said endless surface and the bottom surface of said forward wall, the height of the rear edge of said bottom surface above said endless surface being at least about twice the height of the forward edge of said bottom surface above said endless surface.

9. The invention in accordance with claim 8 wherein said reservoir-establishing means is disposed with said bottom surface of said forward wall above said endless surface a suflicient distance to cast a membrane no greater than about 100 microns in thickness.

References Cited UNITED STATES PATENTS 1,921,953 8/1933 Stephens 18-15 2,226,186 12/ 1940 Van Derhoef 18-15 2,301,368 11/1942 Carver 18-15 XR 2,313,125 3/1943 Czapek 18-15 2,344,603 3/1944 Czapek 18-15 2,754,544 7/1956 Bicher 18-15 2,775,257 12/1956 Stern et al 18-15 XR 2,965,927 12/ 1960 Crosby et al. 18-15 WILLIAM J. STEPHENSON, Primary Examiner, 

5. IN APPARATUS FOR CONTINUOUS CASTING OF A THINPOROUS MEMBRANE SHEET FROM A FLUID CASTING SOLUTION OF ORGANIC MATERIAL, WHICH APPARATUS INCLUDES A CYLINDRICAL CASTING DRUM HAVING A SMOOTH SURFACE, MEANS ROTATABLY MOUNTING SAID DRUM ON A HORIZONTAL AXIS, MEANS FOR ESTABLISHING A SUPPLY RESERVOIR OF THE FLUID CASTING SOLUTION ABOVE SAID DRUM SURFACE, SAID RESERVOIR-ESTABLISHING MEANS HAVING A REAR WALL AND A FORWARD WALL SPACED APART AT THEIR LOWER PORTIONS TO PROVIDE AN OPENING THEREBETWEEN, MEANS FOR SPACING SAID FORWARD WALL A PREDETERMINED DISTANCE ABOVE SAID DRUM SURFACE SO THAT A THIN FILM OF CASTING SOLUTION FLOWS ONTO SAID DRUM SURFACE IN THE DIRECTION TOWARD WHICH SAID DRUM ROTATES, AND MEANS FOR ROTATING SAID DRUM AT A PREDETERMINED SPEED, THE IMPROVEMENT WHICH COMPRISES SAID RESERVOIR-ESTABLISHING MEANS BEING DISPOSED SO THE HYDRAULIC HEAD OF SAID RESERVOIR BEARS DIRECTLY UPON SAID DRUM SURFACE THROUGH SAID OPENING, THE BOTTOM OF SAID FORWARD WALL BEING SMOOTH AND SUBSTANTIALLY FLAT AND BEING LOCATED ON THE UPHILL SIDE OF THE CREST OF SAID DRUM SURFACE AND THE BOTTOM SURFACE OF THE FORWARD WALL, AND THE HEIGHT OF THE REAR EDGE OF SAID BOTTOM SURFACE ABOVE SAID DRUM SURFACE BEING AT LEAST ABOUT TWICE THE HEIGHT OF THE FORWARD EDGE OF SAID BOTTOM SURFACE ABOVE SAID CASTING DRUM SURFACE. 